I used Ubuntu until PAE became required and then switched to either Puppy or DSL (tried them both, honestly don’t remember which I stuck with). Eventually got a new computer and used Fedora and Arch (btw) for years. I’ve recently switched to Debian on a machine I just don’t wanna be arsed with worrying about breaking.
Deleting my grub config instead of editing it. Fortunately that’s pretty easy to recover from, just annoying.
F’ed up installing graphics driver and had to reset everything from another TTY, also just annoying.
Chose the wrong permissions or path on a chmod call and locked out a big party of the system. I think that was during a setup though, so I just started from scratch again.